The present invention relates to an image recording apparatus and method for recording an image on an edge-lit holographic stereogram.
A technique using a holographic stereogram is known as a method for recording a three-dimensional image on a recording medium. A holographic stereogram is produced by sequentially recording, on a single hologram recording medium, original images that are a number of images obtained by sequentially photographing an object from different observation points.
Among various kinds of holographic stereograms that are produced in the above manner, a holographic stereogram having a parallax only in the horizontal direction is produced by sequentially recording, on single hologram recording medium, as rectangular elemental holograms, original images that are a number of images obtained by sequentially photographing an object from different observation points.
For example, as shown in FIG. 1, a plurality of original images 601a-601e obtained by sequentially photographing an object 600 from observation points that are different in the horizontal direction are sequentially recorded on a hologram recording medium 602 in the form of rectangular elemental holograms.
Since pieces of image information obtained by sequential photographing from observation points that are different in the horizontal direction are sequentially recorded in the horizontal direction on a holographic stereogram in the form of elemental holograms, when an observer sees the holographic stereogram through both eyes, two-dimensional images perceived through his right and left eyes are slightly different from each other. As a result, the observer senses a parallax, that is, a three-dimensional image is reproduced according to the principle of stereoscopic vision.
Incidentally, in usual holograms, an illumination light source for reproducing a three-dimensional image is spatially distant from a hologram. Therefore, in usual holograms, a wide space is needed for reproduction and to enable reproduction under the optimum conditions a hologram and an illumination light source should be located so that their positional relationship satisfies given conditions. The same thing applies to a holographic stereogram that is constituted of a plurality of elemental holograms.
If an illumination light source and a hologram are integral with each other, no space for illumination is necessary and hence the apparatus can be miniaturized. Further, since the positional relationship between the hologram and the illumination light source is always the same, reproduction can always be performed under the optimum conditions. This is realized by an edge-lit hologram with which recording and reproduction are performed by using a recording medium that is stuck to a transparent light introduction block.
To produce, according to the edge-lit scheme, a transmission-type hologram for reproducing a three-dimensional image with light that passes through a recording medium, a hologram recording medium 611 is stuck to one surface 610a of a light introduction block 610 that is made of a transparent material, such as glass or plastic, having a proper thickness, as shown in FIG. 2. Usually, to prevent total reflection of light, the hologram recording medium 611 is stuck to the light introduction block 610 via an index matching liquid 612. While an object beam 614 coming from an object 613 is applied to the hologram recording medium 611 through the other surface 610b of the light introduction block 610, a reference beam 615 is applied to the hologram recording medium 611 through an end face 610c of the light introduction block 610. As a result, a transmission-type edge-lit hologram is produced.
For reproduction of a transmission-type edge-lit hologram that has been produced in the above manner, as shown in FIG. 3, in a state that a hologram 621 is stuck to one surface 620a of a light introduction block 620 for introduction of a reproduction illumination beam via an index matching liquid 622, a reproduction illumination beam 623 is applied to the hologram 621 through an end face 620b of the light introduction block 620. In passing through the hologram 621, the light is diffracted by the hologram 621. A resulting diffraction beam 624 causes a reproduction image 625, which is viewed by an observer 626.
On the other hand, to produce, according to the edge-lit scheme, a reflection-type hologram for reproducing a three-dimensional image with light that is reflected by a recording medium, a hologram recording medium 632 is stuck to one surface 630a of a light introduction block 630 via an index matching liquid 631 as shown in FIG. 4 in the same manner as in the case of producing a transmission-type edge-lit hologram. In the case of the reflection type, while an object beam 634 coming from an object 633 is applied to the hologram recording medium 632 from the side where the hologram recording medium 632 is stuck, a reference beam 635 is applied to the hologram recording medium 632 through an end face 630b of the light introduction block 630. As a result, a reflection-type edge-lit hologram is produced.
For reproduction of a reflection-type edge-lit hologram that has been produced in the above manner, as shown in FIG. 5, in a state that a hologram 641 is stuck to one surface 640a of a light introduction block 640 via an index matching liquid 642, a reproduction illumination beam 643 is applied to the hologram 641 through an end face 640b of the light introduction block 640. When reflected by the hologram 641, the light is diffracted by the hologram 641. A resulting diffraction beam 644 causes a reproduction image 645, which is viewed by an observer 646.
In edge-lit holograms as described above, by virtue of the structure in which the light source of a reproduction illumination beam and the light introduction block are integrated with each other, the reproduction optical system can be miniaturized and reproduction can always be performed under the optimum conditions. Further, edge-lit holograms have a feature that because of a large incident angle of a reproduction illumination beam, there does not occur an event that an image is reproduced by external light entering the light introduction block. Because of this feature, edge-lit holograms are increasingly used in different fields such a head-up display apparatus where reproduction of an image by light from the sun, for instance, is not preferable.
Various studies are now being made of colorization of holograms. Two methods are mainly known for this purpose, Lippmann holography and rainbow holography.
In Lippmann holography, a color hologram is produced by causing wavelength selectivity in such a manner that a reflection-type hologram is produced by applying an object beam and a reference beam through different surfaces of a hologram recording medium. However, to produce a color hologram according to Lippmann holography, it is necessary to use three laser beams or performing a swelling treatment on a recording medium. Therefore, the Lippmann holography is not appropriate for small, inexpensive image recording apparatuses.
On the other hand, the rainbow holography has an advantage that a color hologram is produced by a single laser beam and hence is suitable for small, inexpensive image recording apparatuses. The rainbow holography can be applied to a holographic stereogram by, for example, recording rainbow holograms on a hologram recording medium as elemental holograms. In the following description, a holographic stereogram produced in such a manner that each elemental hologram is a rainbow hologram is called a rainbow-type holographic stereogram.
However, various problems occur when attempts are made to produce a one-step holographic stereogram by utilizing the rainbow holography. The one-step holographic stereogram means a holographic stereogram that is a recording medium on which an interference fringe of an object beam and a reference beam is recorded directly. Another method of producing a holographic stereogram is to obtain it by transferring, to another recording medium, an interference fringe of an object beam and a reference beam that is recorded on a recording medium. A holographic stereogram produced in this manner is called a two-step holographic stereogram.
For example, an optical system shown in FIGS. 6A and 6B is used in producing a one-step holographic stereogram by utilizing the rainbow holography. FIG. 6A is a top view of an optical system of the entire image recording apparatus which produces a rainbow-type one-step holographic stereogram, and FIG. 6B is a side view of a portion handling an object beam of the optical system of the image recording apparatus.
As shown in FIGS. 6A and 6B, in this image recording apparatus, a reference beam La and an object beam Lb are applied to a recording medium 700 from the same side. A first lens 701 and a second lens 702 are provided in an object beam Lb side optical path. The first lens 701 focuses the object beam Lb in a direction in which an intended holographic stereogram should have a parallax indicated by arrow a. The second lens 702 focuses the object beam Lb in a direction in which the intended holographic stereogram should not have a parallax. In the following description, the direction in which an intended holographic stereogram should have a parallax is called a parallax direction, and the direction in which an intended holographic stereogram should not have a parallax is called a non-parallax direction indicated by arrow b.
In the image recording apparatus shown in FIGS. 6A and 6B, the reference beam La is applied at a predetermined angle from the same side as the object beam Lb. The object beam Lb is focused (i.e., given focusing action) in the non-parallax direction by the second lens 702 and then focused in the parallax direction by the first lens 701. The reference beam La and the object beam Lb interfere with each other on the hologram recording medium 700, whereby an elemental hologram is formed on the hologram recording medium 700.
In a holographic stereogram obtained by forming each elemental hologram in the above manner, each elemental hologram is a rainbow hologram. Therefore, such a holographic stereogram can be reproduced by white light as illustrated in FIG. 7.
Assume that, as shown in FIG. 7, a reproduction image is viewed in such a manner that while a white illumination beam 710 is applied to a holographic stereogram 711 at the same angle as a reference beam La was applied in forming each elemental hologram, the pupils 712 of an observer are located at the focal point of an object beam that was used in forming each elemental hologram. At this time, only that beam 713 of reproduction beams diffracted by each elemental hologram, which has the same wavelength component as in the recording, is incident on the pupils 712.
On the other hand, a beam 714 having a longer wavelength than the beam used in the recording is focused at a position different from the position of the pupils 712. Similarly, a beam 715 having a shorter wavelength than the beam used in the recording is focused at a position different from the position of the pupils 712. In this manner, only the reproduction beam having the particular color component is incident on the pupils 712 of the observer, who views an reproduction image with a low degree of blur due to color dispersion.
Incidentally, in the image recording apparatus of FIGS. 6A and 6B, the first lens 701 that is next to the recording medium 700 needs to be disposed at a position very distant from the recording medium 700 to prevent the reference beam La from entering the first lens 701. However, usually the first lens 701 should be a lens having a large converging angle. Therefore, to dispose the first lens 701 at a position distant from the recording medium 700, it is necessary to make the diameter of the first lens 701 very large, which is a problem.
Further, to produce a rainbow-type holographic stereogram, it is necessary to focus an object beam not only in the parallax direction (as in the case of producing an ordinary holographic stereogram) but also in the non-parallax direction. To this end, the image recording apparatus of FIGS. 6A and 6B has the second lens 702 for focusing the object beam Lb in the non-parallax direction on the side upstream of the first lens 701 for focusing the object beam Lb in the parallax direction, that is, on the side of a display device 703 for displaying an image to be recorded. However, if the second lens 702 is disposed upstream of the first lens 701, aberrations occur in the first lens 701, causing a problem that elemental holograms are not exposed sharply.